I should mention since looking into it more this thread over which has had posts about this issue for the last 2 years clearly shows the issue at PN.
https://community.plus.net/t5/Broadband/198-18-1-x-a...
Although reading the thread it also shows that PN have a fix for it... eventually
I wasn't privy to the conversation that went on between the end user and PN, I just know the result was that the only option was buy a router, or sign up to a new 24 month contract and get a free router, and that because of that they took the option to just leave for another provider at a much cheaper "new user" price and were supplied a new router with only a 12 month contract.